The significant polarization components of a medium produced by contact with an electric field are first order polarization (linear polarization), second order polarization (first nonlinear polarization), and third order polarization (second nonlinear polarization). On a molecular level this can be expressed in Equation 1: EQU P=.alpha.E+.beta.E.sup.2 +.gamma.E.sup.3 . . . (1)
where
P is the total induced polarization, PA1 E is the local electric field created by electromagnetic radiation, and PA1 .alpha., .beta., and .gamma. are the first, second, and third order polarizabilities, each of which is a function of molecular properties. PA1 P is the total induced polarization, PA1 E is the local electric field created by electromagnetic radiation, and PA1 .chi..sup.(1), .chi..sup.(2), and .chi..sup.(3) are the first, second, and third order polarization susceptibilities of the electromagnetic wave transmission medium.
.beta. and .gamma. are also referred to as first and second hyperpolarizabilities, respectively. The molecular level terms of Equation 1 are first order or linear polarization .alpha.E, second order or first nonlinear polarization .beta.E.sup.2, and third order or second nonlinear polarization .gamma.E.sup.3.
On a macromolecular level corresponding relationships can be expressed by Equation 2: EQU P=.chi..sup.(1) E+.chi..sup.(2) E.sup.2 +.chi..sup.(3) E.sup.3 . . . (2)
where
.chi..sup.(2) and .chi..sup.(3) are also referred to as the first and second nonlinear polarization susceptibilities, respectively, of the transmission medium. The macromolecular level terms of Equation 2 are first order or linear polarization .chi..sup.(1) E, second order or first nonlinear polarization .chi..sup.(2) E.sup.2, and third order or second nonlinear polarization .chi..sup.3 E.sup.3.
To achieve on a macromolecular level second order polarization (.chi..sup.(2) E.sup.2) of any significant magnitude, it is essential that the transmission medium exhibit second order (first nonlinear) polarization susceptibilities, .chi..sup.(2), greater than 10.sup.-9 electrostatic units (esu). To realize such values of .chi..sup.(2) it is necessary that the first hyperpolarizability .beta. be greater than 10.sup.-30 esu.
A significant difficulty encountered in finding suitable molecular dipoles for second order polarization effects lies in the molecular requirements that must be satisfied to achieve usefully large values of .beta.. For a molecule to exhibit values of .beta. greater than zero, it is necessary that the molecule be asymmetrical about its center--that is, noncentrosymmetric. Further, the molecule must be capable of oscillating (i.e., resonating) between an excited state and a ground state differing in polarity. It has been observed experimentally and explained by theory that large .beta. values are the result of large differences between ground and excited state dipole moments as well as large oscillator strengths (i.e., large charge transfer resonance efficiencies).
For .chi..sup.(2) to exhibit a usefully large value it is not only necessary that .beta. be large, but, in addition, the molecular dipoles must be aligned so as to lack inversion symmetry. The largest values of .chi..sup.(2) are realized when the molecular dipoles are arranged in polar alignment--e.g., the alignment obtained when molecular dipoles are placed in an electric field.
Second order polarization (.chi..sup.(2) E.sup.2) has been suggested to be useful for a variety of purposes, including optical rectification (converting electromagnetic radiation input into a DC output), generating an electro-optical (Pockels) effect (using combined electromagnetic radiation and DC inputs to alter during their application the refractive index of the medium), phase alteration of electromagnetic radiation, and parametric effects, most notably frequency doubling, also referred to as second harmonic generation (SHG).
For a number of years the materials employed for achieving second order polarization effects were noncentrosymmetric inorganic crystals, such as potassium dihydrogen phosphate and lithium niobate. Interest in nonlinear optical properties has increased in recent years, driven primarily by the emergence of optical telecommunication, but also stimulated by a broader need to raise optical manipulation capabilities closer to parity with those employed in electronics. This has resulted in an unsatisfied need for higher performance materials.
D. J. Williams, "Organic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities", Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703, postulates mathematically and experimentally corroborates second order polarization susceptibilities in organic dipoles equalling and exceeding those of conventional inorganic dipoles.
Williams reports second order polarization susceptibilities, .chi..sup.(2), achieved with a variety of organic molecular dipoles. The molecular dipoles reported are comprised of an electron acceptor moiety bonded to an electron donor moiety by a linking moiety providing a conjugated .pi. bonding system for electron transfer. Specific electron donor moieties disclosed are dimethylamino, 2- or 4-pyridyl, 2-quinolinyl, and 2-benzothiazolyl. Specific conjugated .pi. bonding systems reported are phenylene and combinations of ethylene (vinylene) and phenylene moieties. Specific electron acceptor moieties disclosed are oxo, cyano, and nitro.
Zyss "Nonlinear Organic Materials for Integrated Optics", Journal of Molecular Electronics, Vol. 1, pp. 25-45, 1985, discloses in FIG. 1 a variety of molecular structures for nonlinear optics and in FIG. 2 varied nonlinear waveguide constructions.
Garito U.S. Pat. No. 4,431,263 discloses nonlinear optical, piezoelectric, pyroelectric, waveguide, and other articles containing a polymer of a diacetylene.
Choe U.S. Pat. No. 4,605,869 discloses a laser frequency converter containing a polymer of the structure: ##STR1## where n is an integer of at least 3 and Y is disclosed to be "nitro, cyano, trifluoromethyl, acyl, carboxy, alkanoyloxy, aroyloxy, carboxymido, alkoxysulfonyl, aryloxysulfonyl, and the like."
Girling, Cade, Kolinsky, and Montgomery, "Observation of Second Harmonic Generation from a Langmuir-Blodgett Monolayer of a Merocyanine Dye,"Electonics Letters, Vol. 21, No. 5, 2/28/85, disclose second harmonic generation with a merocyanine dye Langmuir-Blodgett (hereinafter also referred to as LB) monolayer. A Y-type LB layer assembly is also reported, but without second harmonic signal characteristics.
Neal, Petty, Roberts, Ahmad, and Feast, "Second Harmonic Generation from LB Superlattices Containing two Active Components," Electronics Letters, Vol. 22, No. 9, 4/24/86, disclose Y-type LB films formed of a hemicyanine dye and a nitrostilbene.
Singer, Sohn and Lalama, "Second Harmonic Generation in Poled Polymer Films", Appl. Phys. Lett., Vol. 49, No. 5, 8/4/86, pp. 248-250, discloses placing the azo dye Disperse Red in poly(methyl methacrylate), spin coating on a transparent electrode of indium tin oxide, overcoating with a thin layer of gold, raising the film above its glass transition temperature, applying a poling electric field, and then the film well below its glass transition temperature with the field applied.